The present invention relates to an ultrasonic liquid atomizing apparatus useful for humidifying dried spaces and atomizing medicated liquid.
Conventional ultrasonic liquid atomizers include a free-running Colpitts oscillator that generates high frequency energy in the range between 800 kHz and 2000 kHz and drives a piezoelectric vibrator mounted in energy transfer relation with liquid contained in a chamber. Full-wave rectified, nonfiltered sinusoidal source voltage is derived from a mains supply and a bias voltage is derived from the full-wave source voltage. The oscillator includes a transistor which is powered by the full-wave source voltge and biased by the full-wave bias voltage. The vibrator presents an energy radiating surface which is parallel with the surface of the liquid to maximize the energy transfer efficiency. A liquid sensing switch is provided in the liquid to complete a circuit for coupling the bias voltage to the oscillator when the chamber is filled with liquid, but opens the circuit to prevent the oscillator from being overloaded in the event that the liquid is depleted.
One disadvantage of the prior art apparatus resides in that fact that acoustic energy, initially generated in response to application of a mains supply to the apparatus or in response to reapplication of the bias voltage, is totally reflected from the liquid surface and impinges on the vibrator with a high level of energy. The energy reflected back to the vibrator induces a current in the vibrator. The induced current adds to the current generated by the oscillator with the result that the combined current may exceed the ratings of the vibrator, a transistor of the oscillator and other components of the oscillator.
Previous attempts to eliminate this problem involve starting the oscillator with reduced power and gradually increasing the power until continuous oscillation is attained. One approach, which is considered most relevant to the present invention, employs a zero crossing detector that detects when the apparatus is ready for operation and detects a zero crossover point of a full-wave source voltage to drive a thyristor into conduction. Oscillation starts in response to the thyristor being rendered conductive at the zero-volt point of the full-wave source voltage and gradually increases in amplitude to attain a maximum level within the period of a halfwave of the source voltage. The prior art soft start circuit of this type has a memory and logic circuit to initiate conduction of the thyristor. The use of a zero crossing detector, thyristor and memory results in a costly and complex apparatus.